Lighting fixtures having conduit means for the circulation of a heat transfer fluid in heat transfer relationship with the fixture have been suggested, U.S. Pat. No. 3,424,233, and offer significant advantages in air conditioning systems. A substantial amount of heat energy is necessarily released from lights within an air conditioned building. This heat energy is at a comparatively high level, so that a major portion thereof can be absorbed by lighting fixtures associated with light sources, and transferred to a circulated liquid heat transfer fluid at a comparatively high temperature. Under cooling conditions, the comparatively high temperature heat transfer fluid can be circulated to a heat exchanger, usually of the indirect type, and the energy absorbed thereto can be discharged from the air conditioning system without the necessity for the use of refrigeration. In addition, it is frequently necessary at any given time to remove heat from one zone of a building that is air conditioned and, at the same time, to add heat to another zone of the same building. Lighting fixtures which include means for transferring the heat energy necessarily associated with lighting to a circulated heat transfer fluid are admirably suited for use in the control of air conditioning systems to meet the changing needs of different zones of a building for addition and removal of heat.
By way of example, the circulating system which supplies a heat transfer fluid to lighting fixtures of the indicated type for a given zone of an air conditioned building can be controlled in response to a temperature sensor which measures instantaneous need of a particular zone of the building for addition or removal of heat to open a suitable valve to enable circulation of the heat transfer fluid when heat removal is required, and to close that valve when addition of heat to the zone in question is necessary to maintain a comfort condition. Such a control system is disclosed in U.S. Pat. No. 3,401,742. A zone temperature control system of this particular type is especially effective if air is circulated from the zone in question to absorb heat from the lighting fixture or fixtures serving that zone when heating is required, and the heated air is then returned to the zone.
It has been found, however, that the light output of a given lighting fixture varies as a function of the temperature at which the light source therein operates. As a consequence of this variation, controlling the circulation of heat transfer fluid to lighting fixtures serving a building zone for the purpose of controlling the temperature of that zone, causes variations in the lighting intensity of those lighting fixtures. In general, an increase in the temperature at which a light source operates causes a decrease in lighting intensity. Periodic variations in the lighting output of fixtures is undesirable.